What is a germ?

A scientist uses a special microscope to look at bacteria. Photo from CDC.govWe know that germs can make us sick, but what exactly are germs?

To learn about germs, you have to learn some important scientific words. A germ is a pathogen. A pathogen is a microbe that causes disease. A microbe is a living thing that is too small to see without a microscope. Scientists use microscopes to discover different types of pathogens.


Bacteria are not animals, but they are living things. Bacteria are very small. Thousands of bacteria could fit on a pencil eraser. Bacteria live everywhere. They live in the soil, in plants, in animals and even on your computer keyboard. Bacteria also come in different shapes. Under a microscope, you might see bacteria shaped like circles, rods and spirals.

Most bacteria do not make people sick. Some bacteria even help us. Bacteria actually live in your body and help you digest food and stay healthy. If you did not have these good bacteria, the bad bacteria could take over and make you sick.

This is a group of Salmonella bacteria. Salmonella is a rod-shaped bacterium that can make people sick. Photo from CDC.govBad bacteria are pathogens. These bacteria try to live in parts of your body where they do not belong. Some kinds of bacteria also give off toxins. Toxins are harmful substances that can make you sick. When bacteria make you sick, you have an infection.

The body can sometimes fight infections by itself. The body has a special system called the immune system. The immune system tells your body that a pathogen does not belong there. Special cells from the immune system will attack the pathogen. If your immune system cannot kill bad bacteria, a doctor might give you antibiotics. Antibiotics are special medicines that kill bacteria.


There are many different kinds of fungi. The mushrooms on pizza are one kind of fungus. When bread grows mold, that mold is another kind of fungus. Most fungi do not make people sick.

This is a kind of fungi called Aspergillus. Asperillis fungi are usually harmless, but some can cause disease in humans and animals. Photo from CDC.govFungi can be big, like plants, or small, like bacteria. Many kinds of fungi spread by sending out very tiny spores. Spores are sort of like seeds. A new fungus grows when a spores lands in a suitable environment.

Sometimes small fungi try to grow in people. Athlete's foot and ringworm are two kinds of skin infections caused by fungi. These two infections can make your skin itch or hurt, but they are not life-threatening. In the United States, life-threatening fungal infections are rare. Many pathogenic fungi have a hard time living inside the human body.

Some fungi can actually help people too. The antibiotics that fight bacteria are made from fungi. Just like good bacteria, good fungi live inside your body and help you stay healthy.


This is an illustration of the influenza virus. This virus is shaped like a ball. Photo from CDC.govYour body is made up of tiny cells that perform special jobs. For example, red blood cells carry oxygen to parts of your body. Viruses make you sick by taking over the cells in your body. Normally, a cell divides and produces more cells. But when a cell is infected with a virus, the cell produces more viruses. The virus tricks the cell into become a virus factory.

Like bacteria and fungi, viruses live everywhere. Luckily, many viruses do not affect humans. When a human does get a viral infection, the immune system will try to fight it. The disease influenza, also called the "flu," is caused by a viral infection. Antibiotics do not fight viral infections, but scientists have developed some antiviral drugs.

Learn how to protect humans and animals from pathogens